This invention relates to a fabricating process for the manufacture of heavy wall UO pipe, and more particularly to a process for fabricating plates having a thickness of 19 mm and greater.
The UO process is a typical pipe manufacturing process using electric welding to join the edges of a plate bent into a circular cross-section. This process comprises crimping or edge-planing the steel plate or workpiece, which is then shaped into a U-shaped cross-section, shaped into an almost closed circle by the O-ing press, welded, and finished to the desired shape and size by expanding or contracting.
Recently steel pipes with greater wall thickness and higher tensile strength, such as those having a wall thickness of 19 mm or more and conforming to API standard (specifying a yield point of 45.6 kg/mm.sup.2 and above) or higher, have come to be required.
Despite its high production rate, the UO process involves the following problems in fabricating heavy wall pipes:
(1) Excessive Peaking after O-ing PA0 (2) Inadequate circular shape after O-ing PA0 (3) Increase in O-ing press power
Inadequate bending by the O-ing press develops peaking or butt projection 4 on the O-shaped workpiece 3, as shown in FIG. 1. The amount of peaking, which appears as out of roundness, as seen in FIG. 1) tends to grow excessive especially on heavier wall pipes. This peaking 4 remains after welding. Even expanding cannot fully remove the peaking on heavier wall pipes, resulting in ill-shaped products. Excessive peaking causes a large bending moment during expanding, which exerts heavy strain in the vicinity of the weld and therefore reduces the toughness of the pipe.
From time to time, the O-ing operation fails to give a satisfactorily circular cross section to the workpiece, making its diameter greater than desired in one part and smaller in another, or making it look like an ellipse. This out of roundness can be corrected to a considerable extent by applying a greater expanding force. But the intensified expansion is undesirable because it entails increased strain, causing deterioration of toughness. The expander power must be increased, too.
In principle, the above problems (1) and (2) can be solved by increasing the power of the O-ing press to apply adequate compressive strain during the O-ing process. This solution, however, calls for a huge O-ing press power. The inventors have found that O-ing 38 mm thick steel plate of API X-70 class steel into a 48-in. (1219 mm) dia. pipe requires a load of approximately 3800 tons per meter of length. Accordingly, fabrication of 18 m long pipe requires an O-ing press power of approximately 70,000 tons. Such a huge press is commercially unavailable. Even if it were available, it would be prohibitively costly.
The crimped plate is first subjected to the U-ing and then to the O-ing operations. The butting edges of the O-shaped workpiece are tack-welded. The clearance .delta. (hereinafter called the gap) between the edges of the O-shaped workpiece 3, shown in FIG. 1, has a great influence on welding efficiency and weld quality. When no gap is left, foreign matter (dust, oil, etc.) left between the edges results in poor welding. When the gap is too large, weld crack and other defects increase. To avoid these problems, a tack welding machine is commonly provided with a mechanism to forcibly produce a gap before welding and an external restraining mechanism to forcibly reduce the gap to zero when proceeding with the welding. With heavy wall pipes, however, the load on these mechanisms becomes extremely large since it increases in proportion to the square of plate thickness. The load thus created is too large for the conventional mechanisms to bear. Consequently, welding efficiency and weld quality are impaired. Providing an appropriate gap after O-ing is clearly important and effective for the increase of welding efficiency and product quality. Especially in the manufacture of pipes with a wall thickness of 19 mm and heavier, the necessity of gap control becomes extremely important, and the permissible tolerance for the desired gap extremely limited. For wall thickness of 25.4 mm and heavier, gap control is indispensable.
This gap varies in a complicated way with the workpiece properties, especially thickness and strength, and the O-ing conditions, especially the O-ing load. The following are typical known gap control methods:
(a) To provide a key on the top O-ing die so that fabrication is performed with the key held between the butting edges.
(b) To control the O-ing load.
(c) Combination of (a) and (b).
All these methods control the gap by modifying the O-ing conditions. But as a practical matter they are inapplicable to the heavy wall pipes, the manufacture of which constitutes the object of this invention, for the following reasons:
(i) The heavy wall pipes will probably break the key employed by the method (a).
(ii) The O-ing operation is required to form the workpiece close to a true circle. This requirement calls for a large enough O-ing load. Meanwhile, the gap has a general tendency to decrease with an increasing O-ing load and plate thickness. Accordingly, in the manufacture of the heavy wall pipes, the method (b) has little or no freedom to harmonize the requirement for fabrication of the workpiece with as little out-of-roundness as possible, i.e. the application of a sufficiently large O-ing load, with the requirement for providing an appropriate gap, i.e. a limitation of the O-ing load. To increae this freedom, the O-ing press capacity must be increased to a great extent, which entails great impairment of O-ing efficiency.
(iii) With the narrowing gap tolerance limits for the heavy wall pipes, none of the above-described methods can satisfy varying requirements for different material plate thicknesses and strengths and pipe diameters.